Optical data storage systems have employed optical disks, such as magneto-optical, ablative or other types of optical disks. Such optical disks have substrates with axial dimensions that limit areal recording densities. Efforts are continuing to increase the areal densities of such optical disks. Such increased areal densities require shorter laser beam wavelengths, larger numerical apertures in the optical system and a smaller track pitch. Such changes result in increased difficulties in disk manufacture. Such manufacturing difficulty increases disk costs. Since data storage is cost competitive, a different approach is required.
One approach for increasing areal density of optical disks is to decrease the axial thickness of the transparent substrate. Such thinner substrates result in shorter optical paths within the substrates to and from an embedded reflective magneto optical recording layer. Current transparent substrates are relatively thick, i.e. 1.2 millimeters (mm),for example, Reducing the axial thickness of the transparent substrate creates an incompatibility problem between original optical disks (termed herein thick-substrate disks) and newer axially thinner substrates having an axial thickness of about 0.6 mm. An optical system designed to operate with (read from, erase and write to) the thin-substrate optical disks results in excessive spherical aberration from thick-substrate disks preventing successfully reading from or writing to such thick-substrate optical disks. It is highly desired to provide thin-substrate optical disks for achieving greater areal recording densities while maintaining capability to access data recorded in thick-substrate disks. Such desire is termed "backward compatibility". As it turns out, read access to the older thick disks satisfies most backward compatibility requirements, i.e. maintain access to old data. It is also desired to enable automatic controls for facilitating such backward compatibility without requiring separate optical disk drives for the two types of optical disks.
An effective low-cost method and apparatus for reducing the effects of spherical aberration of reflected laser beams from thick-substrate optical disks (reading such disks) in optical devices designed for writing to and reading from thin-substrate optical disk is needed.